The invention relates generally to the field of automated focus adjustment for imaging optical systems, and in particular to autofocus and tilt adjustment for microscope systems.
Magnetic recording heads are manufactured using processes and techniques similar to those of integrated circuit fabrication. Deposition, lithography and etch processes are performed on AlTiC ceramic wafers to form an array of thin film read/write transducers. Wafers are then sliced into bars, whereupon the air bearing surfaces are formed by precision machining and lapping. Finally the bars are cut into individual sliders or heads. At this point, it is desirable to inspect each head for defects or damage before they are attached to a suspension and subsequently integrated into the disk drive assembly. Optical inspection of recording head surfaces and read/write transducers must be capable of accommodating a wide variety of structural and material properties. Air bearing surfaces are complex 3-dimensional structures fabricated from granular ceramic composites while head read/write transducers are micron-size metal/insulator structures. For such applications, optical head inspection systems typically comprise semi-automated microscopes that require a human operator to visually detect and classify defective heads and often to operate the microscope itself. Over one billion heads per year are fabricated and inspected in this fashion. Clearly, it would be highly desirable to conduct optical head inspection in a completely automated manner.
A major component in an automated optical inspection microscope is the autofocus system, which should be fast, accurate and robust. Generally such systems comprise a focus sensor and a focus driver. The sensor provides a focus error signal that is indicative of variations of the optical system focal plane from the actual object plane. The driver is usually a motor and stage combination that adjusts either the object or the focal plane in response to the focus error signal.
Microscope autofocus systems typically utilize a focus figure of merit based on image intensity or image contrast as the focus sensor signal. For example, U.S. Pat. No. 5,483,055 by Thompson et al. discloses a microscope autofocus system that uses a laser beam focussed through the microscope objective onto the surface of interest. The intensity of the reflected beam is measured by a photodetector and continuously monitored to achieve a maximum as the object is positioned near the focal plane of the microscope. While such systems may be adequate for inspecting smooth, reflecting surfaces, they may be limited in speed when large variations in topography or reflectivity are encountered.
Another common autofocus technique utilizes some criterion for image contrast such as the sharpness of a well-defined object edge. In optical inspection microscopes, in particular for those used in IC manufacturing and inspection, a pattern is projected onto the object surface through the microscope objective, and the superposed image analyzed for pattern contrast. U.S. Pat. No. 4,725,722 by Maeda et al., U.S. Pat. No. 4,945,220 by Mallory et al. and U.S. Pat. No. 5,604,344 by Finarov each disclose microscopes using projected pattern contrast for autofocus control. Again, such systems may be adequate for inspecting smooth surfaces such as semiconductor wafers, but are not readily usable for rough granular, surfaces where the superposed image may have a low degree of contrast.
Still another effective method for microscope autofocus is triangulation where oblique illumination of a surface of interest produces a specular reflected beam that shifts in response to changes in the sample position. Position sensitive detectors are placed in the return path of the beam to detect the displacement. Triangulation autofocus systems are disclosed in U.S. Pat. No. 4,577,095 by Watanabe and U.S. Pat. No. 5,136,149 by Fujiwara et al. In particular, U.S. Pat. No. 5,136,149 discloses a triangulation system for autofocus that can also correct for tilted test surfaces. Autofocus and tilt is desirable when a large test surface is warped or curved such as may occur in semiconductor wafer manufacturing. The method for autofocus and tilt disclosed by Fujiwara et al. utilizes triangulation from a single point on the test surface. For complex test surfaces having facets and other structures, a single test point may not provide adequate tilt information for the majority of the surface.
For automated head inspection, it would be highly desirable to employ an autofocus and tilt system that can accommodate air-bearing surfaces presented in a variety of orientations including tilted surfaces.
It is an object of the present invention to provide a fast, accurate and robust autofocus system for an inspection microscope. Another object of the present invention is to provide a combination autofocus and tilt system for an inspection microscope. Yet another object of the present invention is to provide an autofocus and tilt system capable of accommodating complex test surfaces such as magnetic recording heads.
In accordance with a first aspect of the present invention, an autofocus system in an inspection microscope utilizes a light pattern projected onto a test surface through the microscope objective. An image of the light pattern on the test surface is recorded and analyzed to determine a focus error. The light pattern image is analyzed in portions to determine specific properties of the respective portions. In a basic embodiment of the present invention, the positions of respective portions are determined and compared to calibration data to determine a focus error. The focus error is sent to a motion control system for applying a focus correction to an adjustable microscope stage. In a preferred embodiment of the present invention, a tilt error is also derived from an analysis of the light pattern image.
In accordance with a second aspect of the present invention, an autofocus apparatus for a microscope comprises a translatable stage for positioning a test surface in opposition to a microscope objective and a light pattern generator for projecting a light pattern onto the test surface. The autofocus apparatus further comprises an imaging system for recording an image of the light pattern on the test surface and a processor for analyzing the light pattern image. In accordance with the present invention, the processor analyzes portions of the light pattern image to determine a focus error for the test surface. In a basic embodiment, the processor determines the relative positions of respective portions of the light pattern image and compares these positions to calibrated positions to determine a focus error. The autofocus apparatus further comprises a motion control system to apply a focus correction to the translatable stage. In a preferred aspect of the present invention, the processor also determines a tilt error from an analysis of the light pattern image and a tilt correction is applied by the motion control system to a rotatable stage.
In accordance with yet another aspect of the present invention, a method for automatically focussing a test surface in a microscope comprises projecting a light pattern onto a test surface positioned near the focal plane of the microscope. An image of the light pattern on the test surface is recorded and analyzed by a processor to determine the positions of respective portions of the light pattern image. Comparing those positions to calibration data determines a focus error and preferably also a tilt error. The error signals are sent to a motion control system and corrections are applied to a microscope stage holding the test surface.
These and other objects and aspects of the present invention will become more apparent upon considering the following detailed description and the accompanying figures.